docs: def get_site_type_dummy_variables(self, sites):
          """
          Binary rock/soil classification dummy variable based on sites.vs30.
  
          "``S`` is 1 for a rock site and 0 otherwise" (p. 1201).
          """
          is_rock = np.array(sites.vs30 > self.NEHRP_BC_BOUNDARY)
          return is_rock